Local anesthetics are drugs that can reversibly block generation and transmission of sensory nerve impulses at local administration position, and provide pain relief for local tissues under conditions of keeping people or animals awake. The action of local anaesthetics is related to the diameter of nerve cells or nerve fibers, as well as anatomical features of nerve tissues. In general, nerve fiber terminals, ganglias, and synapses of central nervous system are the most sensitive to local anesthetics, and smaller-diameter fibers are more easily blocked compared with those larger fibers. Local anaesthetics can act on postganglionic fibres of unmyelinated sympathetic and parasympathetic nerves at low concentration. When acting on mixed nerves, persistent dull pain firstly disappeared, then transient sharp pain, following by loss of cold sensation, warm sensation, touch sensation, pressure sensation, as well as occurrence of motor paralysis. Currently, it is well-known that the action mechanism of local anesthetics is blocking voltage-gated Na+ channels of nerve cell membranes, then producing conduction block and local anesthetic effects.
In general, effects of local anesthetics limit to the administration position, and then the effect is fast lost as diffusion of drugs. If realizing long-acting local anesthetic effects is required, dosage of drug need further increase, except for optimizing molecular structures of local anesthetics. At present, local anesthetics clinically used are all molecules without electric charges, and can just exert local anesthesia and analgesia up to 8 hours. Thus, that can not satisfy demands of long-acting local anesthesia exceeding 72 hours. Consequently, there is an urgent need for a new type long-acting local anesthesia with more than 72 hours effects.
Most current local anesthetic drugs often contain at least one N atom from tertiary amine, and further alkyl substitution can produce corresponding quaternary ammonium salts, allowing the molecules have electric charges, thus it is not easy to pass through cell membranes. The ethyl quaternary ammonium salts of N-diethylaminoacetyl-2,6-dimethylanilines, called QX314, is a quaternary ammonium compound early reported with local anesthetic effects. However, due to the strong polarity of QX314 molecules, it can not pass through cell membranes and fast produce strong local anesthetic effects. Thus, QX314 can not be directly used in clinic. But QX314 shows a significant inhibitory on the target points Na+ channels situated in the inside of cell membranes. Once passed through membranes, QX314 can potently inhibit Na+ channels in the membranes, and QX314 in cell membranes hardly diffuses to out of cells, thus lasting anesthetic action can be obtained (Courtney K R. J Pharmacol Exp Ther. 1975, 195:225-236). Currently, many researches indicate QX314 can get into cell membranes, and produce long-time anesthesia (Craig R. Ries. Anesthesiology 0.2009; 111:122-126). Recent investigation has shown that with the addition of surface-active agent or with the help of forming micelles, QX314 can be assisted to enter membranes and cause local anesthetic actions lasting more than 8 hours (Daniel S. Kohane, PNAS. 2010; 107: 3745-3750).